Typical electric devices used in the radio frequency (RF) range of 500 MHz to 100 GHz in mobile phone, onboard radar, and the like include variable capacitance and switch.
Typically, a variable capacitance element has a structure including a fixed electrode and a movable electrode, wherein the movable electrode is displaced to change the capacitance. In order to prevent short circuiting, the surface of the fixed electrode is, for example covered with a dielectric insulating film. A switch can be formed by not covering the surface of the fixed electrode with a dielectric film and allowing the movable electrode to contact with the fixed electrode. The movable electrode can be piezoelectrically or electrostatically displaced. Reduction in size and weight is requested for a mobile electric device, and variable capacitance elements and switches using MEMS (micro electro-mechanical system) have been developed.
Such a structure is known wherein a fixed electrode is formed on the surface of a support substrate, and above the fixed electrode, a movable electrode opposing the surface of the fixed electrode is supported with an intervening flexible beam or the like. The capacitance is altered by controlling the distance between the electrodes. (See for example, JPA No. 2006-147995.)
FIG. 16A illustrates an example structure of variable capacitance device. The variable capacitance device is made of a variable capacitor having a parallel plate structure wherein one of the two electrodes is movable, and a casing for sealing the variable capacitor therein.
More specifically, a fixed electrode 103 in the form of a plane and anchors 106 for supporting a movable electrode are formed on a surface of a semiconductor substrate 101 such as a silicon substrate via an insulation layer 102. The anchors 106 support a plane-shaped movable electrode 104 above the fixed electrode 103 via U-shaped flexible beams 105. The flexible beams 105 have U-shaped beams disposed on the same plane as the movable electrode 104, and allow displacement in the normal direction to (the thickness direction of) the plane. Casing including side wall 110 and ceiling 111 is formed to surround the variable capacitor. The casing enables sealing the variable capacitor in an atmosphere such as an inert gas atmosphere of rare gas or in a reduced pressure atmosphere. When the casing is formed of metal, electrical shielding of the variable capacitor can be made.
When a voltage V is applied between the fixed electrode 103 and the movable electrode 104, the movable electrode 104 is attracted to the fixed electrode 103 by electrostatic force. When the movable electrode 104 is displaced to the side of the fixed electrode 103, the flexible beams 105 are distorted, and there is generated force of returning the movable electrode 104 in reverse direction by the restoration force proportional to the amount of displacement. The movable electrode 104 is displaced to a position where the electrostatic force and the restoration force are balanced, and the movable electrode 104 is retained at the balanced position as long as the voltage V is applied.
When the voltage V is reduced to zero, the movable electrode 104 returns to its original position by the restoration force of the flexible beam 105. Accordingly, the capacitor formed of the fixed electrode 103 and the movable electrode 104 functions as a variable capacitor whose electrostatic capacitance can be controlled by the applied voltage V.
FIG. 16B is a cross sectional view illustrating another example structure of variable capacitance element. A plane-shaped fixed electrode 103 is formed on a surface of a semiconductor substrate 101 such as a silicon substrate, via an insulation layer 102. An insulation layer 112 is formed on the insulation layer 102 to cover the fixed electrode 103. Anchors 106 are formed on the insulation layer 112, and support a plane-shaped movable electrode 104 above the fixed electrode 103 covered with the insulation layer 112 via flexible beams 105. Casing including side wall 110 and ceiling 111 is formed to surround the variable capacitor. Short-circuiting between the electrodes is avoided since the surface of the fixed electrode 103 is covered with the insulation layer 112.
In the case when the fixed electrode is covered with a dielectric insulation film, the dielectric film m be charged up after repeated on and off operations. This may result in the sticking phenomenon, in which separation of the movable electrode from the dielectric film becomes difficult. This problem has not been solved despite some countermeasure proposals including use of particular driving waveform.
When the envelope of radio frequency signal is modulated by signal waveform and is applied to the movable electrode, there may occur a phenomenon called self-actuation in which the movable electrode is moved by difference in the voltage based on the signal waveform. One method for preventing the self-actuation is to increase the driving voltage in accordance with the power of the introduced signal. Increasing the driving voltage causes easier occurrence of the sticking phenomenon. Also, a voltage boost circuit may be required to obtain higher voltage.
The electrode of a capacitor element can be formed not only in direction parallel with the substrate surface, but also in perpendicular to the substrate surface (for example, JPA No. 2001-304868). For example, a variable capacitance is formed by using an SOI (silicon-on-insulator) substrate having an active (single crystal) silicon layer formed above a support (single crystal) silicon substrate via a silicon oxide layer serving as a binding layer, and processing the active silicon layer to form a variable capacitance having an electrode perpendicular to the substrate surface.
The active silicon layer is doped with an impurity such as phosphorus or boron to reduce the resistance of the active silicon layer. A resist mask is formed on the active silicon layer. The active silicon layer is etched by reactive ion etching or the like to leave anchors, various comb-shaped electrodes, various pads, and the like on the silicon oxide layer. The comb-shaped electrodes are coupled to form a capacitance. The electrodes are formed perpendicular to the support silicon substrate.
The silicon oxide film can be removed by selective etching by hydrofluoric acid aqueous solution or the like to separate the active Si layer from the support Si substrate to provide freedom of displacement. Oscillator, beam, comb-shaped electrode, and the like can be formed. Aluminum or the like can be deposited on various pad to form electrode pad. Parts formed above the support substrate are formed of low resistance layer insulated from the support substrate, and oscillator, beam, comb-shaped electrode, and the like are located floating above the support substrate, and are oscillatably supported above the support substrate by the anchors.